As the use of data processing devices and services has become more and more widespread, the ability and use of such devices and services to manage many aspects of users' lives has continued to grow. One significant feature of the widespread use of data processing devices and services has been their increasing use to manage users' social contacts. Large amounts of information relating to users' contacts are stored both locally on users' devices and remotely, such as by social networking websites. Particularly in the case of close or intimate friends, the association of a user's friends with an activity or person contributes to the confidence a user is likely to have in the activity or person. One significant contributor to confidence in an unknown user is to identify friends in common with the unknown user. However, obtaining information identifying common friends presents privacy risks, and also presents risks of resting unjustified confidence in an unknown user who falsely claims to share friends with a user seeking assurances.